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| Dissertation / PhD Thesis/Book | PreJuSER-41885 |
2004
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
Please use a persistent id in citations: http://hdl.handle.net/2128/140
Report No.: Juel-4140
Abstract: The abundance of the higly reactive hydroxyl radical (OH radical) in the troposphere (lower 10 kilometers of the atmosphere) is of great importance for the removal of both naturally occurring trace gases and those brought in by human activities. Ozone (O$_{3}$) is formed in a complex reaction sequence during the OH-induced degradation of volatile organic compounds (VOC) involving nitrogen oxides (NO$_{x}$, NO$_{x}$ = NO + NO$_{2}$) and solar radiation. Elevated levels of ozone mixing ratios in the planetary boundary layer (lower up to two kilometers of the troposphere) represent a serious hazard for human health and vegetation. Isoprene, a volatile organic compound released by vegetation, plays an important role in the formation of ozone. Its high reactivity towards the OH radical in conjunction with its large global emission rate renders isoprene a compound of outstanding importance in the atmosphere. Formaldehyde (HCHO) is a major product from isoprene oxidation and represents a substantial source for HOx radicals (combined OH radicals and hydrogen peroxy radicals (HO$_{2}$)). The atmosphere simulation chamber SAPHIR provides the opportunity to investigate different air masses with defined chemical compositions under conditions (sun light, temperature and pressure) comparable to the atmosphere. An important advantage of SAPHIR is the possibility to study kinetic parameters (for example rate constants) from the gas-phase oxidation of VOC at tropospheric mixing ratios of reactants. The method of sensitivity analysis is employed to develop an experimental design for chamber experiments exploring rate constants in SAPHIR. Chamber sources for gaseous nitrous acid (HONO) and HCHO can have a strong impact on gas-phase chemistry. Parameterisations for these chamber sources were derived from the measured temporal increase of the respective compounds. Time dependent concentration profiles of HCHO from chamber experiments investigating the photochemical removal of HCHO were analysed. Analysis of the chemical balance of HCHO supplied evidence for the correctness of the spectralradiometric method used for the determination of photolysis frequencies of HCHO. The rate constant of HCHO with the OH radical, determined in this work, is within the error limits of the actually recommended value. Based on a current chemical sheme for the degradation of isoprene, which embodies recent kinetic data and reaction pathways, a detailed pattern for the treatment of the isoprene oxidation products methacrolein (MACR) and methyl vinyl ketone (MVK) was developed. In experiments investigating the OH-induced and O$_{3}$-induced oxidation, rate constants were determined for the initial steps, using measured concentrations of isoprene, OH and O$_{3}$. The determined values are in accordance with literature values. In comparison with literature values of OH-induced oxidation of isoprene in the presence of NO$_{x}$, higher product yields were derived for MACR and MVK, while the rate constants of their OH-reactions were lower for about 20 %. The tropospheric relevance of these results was investigated using a photochemical scenario which represents a polluted air mass over a close-to-the-city forest area. It was concluded that MACR and MVK provide an important sink for OH radicals.
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